I am playing my way through Nightmare Diablo 2 on Battle.net closed servers and because of the swarming that's occurring I've decided to bite the bullet and get a hireling to come help out.

My act 1 hireling seemed to be restricted to bow weapons. My act 2 defensive hireling comes with a spiffy cold effect but seeems to be restricted to pole arms, without a shield regardless of the class of pole arm (ie 1 handed pole arms should allow a shield, but don't).

What are the restrictions on hirelings and what other bonuses might I discover?

Additionally to the type restrictions, mercenaries follow the items requirements for Strength, Dexterity and Level.
Back when I played Diablo II, act II Mercenaries were the most common choice due to their buff-auras.

Note that the Ironwolves do not actually physically attack with the sword, so finding one with good casting stats for whatever element you choose is is a better idea than finding one with good weapon damage.